Process of making chloroform-soluble cellulose acetate



Patented Sept. 4, 1928.

"UNITED STATES PATENT OFFICE.-

HARRY LE 13. GRAY AND CYRIL J. STAUD, OF ROCHESTER, NEW YORK. ASSIGNORSTO EASTMAN KODAK COMPANY, OF ROCHESTER, NEW YORK, A CORPORATION OF NEWYORK.

PROCESS OF MAKING CHLOROFORM-SOLUBLE GELL'ULOSE ACETATE.

No Drawing.

This invention relates to processes of making chloroform-solublecellulose acetate, especially processes in which the cellulose is givena pretreatment prior to the main or final acetylation. One object of theinvention is to provide a low cost process of this kind. which willyield cellulose acetate that can be made by subsequent operations intoclear, flexible, transparent films. Another object is to provide aprocess of wide ap plicability to celluloses from different sources.Still another object is to provide a process in which an improvedcatalyst is used that stimulates acetylation without degrading theproduct and serves bothduring the pretreatment and the main or finalacetylation. A further object is to provide a process in which theamount of cellulose sulf-acetates or other sulfur compounds in theproduct is reduced to the minimum. Other objects will hereinafterappear.

We have found that a process meeting the above requirements can becarried out by using a mixed catalyst of sulfuric acid and phosphoricacid in the right proportions, the same catalyst serving both for thepretreatment and for the final acetylation, an appreciable acetylationbeing accomplished during the pretreatment. The phosphoric acid is atleast equal in weight to the sulfuric acid and may even weigh severaltimes as much. We have found that this mixed catalyst functions betterthan sulfuric acid alone. The operations are under better control, thetendency to degrade the cellulose is less, the formation ofsulf-acet-ates is minimized and the final transparent films aredifferent temperatures, being shorter at the upper temperatures. Thelatter are preferably under (3., at which temperature pretreatments of 2to 4 hours generally suffice.

In the'preferred form of our invention Application filed August 25,1927. Serial No. 215,520.

mass before the acetic anhydrid is stirred into it. so that the heatevolved during such additionwill not raise the reaction mass to adangerous temperature. The acetic anhy- (lrid may be added rapidly orslowly at intervals. For uniformity we prefer toadd it so that each partof the cellulosic' material will receive its quota of anhydrid at aboutthe same time as every other part.

It is one of the features of our process that it may be appliedsuccessfully to manyv different kinds of cellulose, such as high gradeclean cotton fibers, cotton fiber tissue paper, such as is especiallyprepared for esterification, surgical cotton wool, cotton linters, andeven carefully prepared and bleached sulfite wood pulp.' These aremerely illustrations of its Wide applicability.

e shall now give one specific example, but it will be understood thatour invention is not limited to the details thus given, except asindicated in the appended claims. Fifty parts by weight of cellulose,say cotton linters which have been purified in the usual way, say by aboil in dilute caustic soda and a short bleach, are mixed with 490 partsof glacial acetic acid containing 3 parts of mixed catalyst. The latteris composed of 2.6 parts by Weight of phosphoric acid strength) and .9parts of sulfuric acid (98% strength). The pretreatment mass, thusobtained, is kept at 38 C. for 4 hours. The ingredients may be broughtto this temperature after mixing, or they may be preheated to thistemperature and then mixed, the latter giving a more easily regula'tabletreatment.

At the end of the pretreatment the mass is brought to a lowertemperature, say room temperature or even 15 C. This can be done byartificial cooling, or by allowing the heat from the mass to pass intothe atmosphere. The time of cooling is not of critical importance and 20r 3 hours has been found convenient.

Into the cooled pretreatment mass there is next stirred 150 to 170 partsby weight of acetic anhydrid (85% strength). This cor responds to about127 to 144 parts by weight of the actual anhydrid. The addition of theanhydrid causes the reaction mass to increase in temperature. Thisoperation is conducted so that the reaction mass finally reaches atemperature within the range hereinabove named, say 42C. If the reactionmass does not reach the required temperature from the evolution of heatwithin, it may be heated by external means so as to bring the massgradually up to the required point. The reaction, with the reagents keptthoroughly mixed, is carried out until the fibers disappear and a clearreaction solution or dope is obtained. Then a test is made of a sampleto make certain that the product is fully soluble in chloroform.

The chloroform-soluble cellulose acetate thus obtained may be hydrolyzedto the acetone-soluble formin any of the known ways, such as by adding amixture of water, mineral acid and acetic acid to the reaction mass andallowing the hydrolysis to proceed at the appropriate temperatures, asis well known. Or the chloroform-soluble cellulose acetate may beobtained from the reaction mixture by precipitating in water andwashing, or by spray drying methods; and then the solidchloroform-soluble cellulose acetate, thus obtained, may be hydrolyzedby treatment with appropriate aqueous acid solutions, ashithertodescribed in the art.

The films produced by subsequent hydrolysis of the chloroform-solublecellulose acetate and final solution in acetone are strong, flexible andbrilliantly transparent and their flexibility is very durable underprolonged tests at the usual testing temperatures. Analysis indicatesthat the amounts of cellulose sulf-acetate and other deleteriouscellulose-sulfur compounds are very low in our product.

Having thus described our invention, what we claim as new and desiretosecure by-"Letters Patent is:

1. In the process of making cellulose-acetate, prctreating the cellulosewith glacial acetic acid containing a mixed catalyst of sulfuric andphosphoric acids in which the latter acid is at least equal to theweight of the former, mixing acetic anhydrid into the pretreated mass tocomplete the acetylation, and conducting the reaction until the productis chloroform-soluble, the same mixed catalyst serving both in thepretreatment and the final acetylation.

2. In the process of making cellulose acetate, pretreating the cellulosewith glacial acetic acid containing a mixed catalyst of sulfuric andphosphoric acids in which the latter acid is from one to five times theweight of the former, said mixed catalyst being less'than 10% of theWeight of the cellulose, and said pretreatment being conducted untilbetween 1 and 3.5% of acetyl has been combined with the cellulose,mixing acetic anhydrid with the pretreated mass to complete theacetylation, said acetylating reaction being carried on until theproduct is chloroform-soluble, the same mixed catalyst serving both inthe pretreatment and the final acetylation.

3. In the process of making cellulose acetate, pretreating the cellulosewith glacial acetic acid containing a mixed catalyst of sulfuric andphosphoric acids in which the latter acid is from one to five times theweight of the former, said mixed catalyst being less than 10% of theweight of the cellulose, and said pretreatment being conducted at atemperature below 40 C. until between 1 and 3.5% of acetyl has beencombined with the cellulose, cooling the mass to at least roomtemperature, stirring in acetic anhydrid to complete the acetylation,said acetylation being conducted at a temperature between 35 and 60 C.until the product is chloroform-soluble, the same mixed catalyst servingboth in the pretreatment and in the final acetylation.

Signed at Rochester, New York, this 19th day of Aug, 1927.

HARRY LE BpGrR/AY. CYBIL J. STAUD.

